Conventionally, blood circuits are disclosed in, for example, Japanese Utility Model Laying-Open No. 58-153841 (Patent Document 1), Japanese Patent Laying-Open No. 61-143069 (Patent Document 2), U.S. Pat. No. 6,526,357 (Patent Document 3), Japanese Patent Laying-Open No. 5-23393 (Patent Document 4), Japanese Patent Laying-Open No. 9-24026 (Patent Document 5) and U.S. Pat. No. 8,092,414 (Patent Document 6).